A Base Transceiver Station (BTS) is a device to connect a mobile station and a communication network, and is operable to relay data transmitted and received therebetween. In the case where the communication network is a mobile phone network, the Base Transceiver Station serves to relay, for example, data transmitted and received between a Radio Network Controller (RNC) of the mobile phone network and a mobile phone terminal. In the case where the communication network is a Local Area Network (LAN), the Base Transceiver Station serves to relay, for example, data transmitted and received between a router of the LAN and a personal computer.
In order to expand an area within which a user can use a mobile station, the number of Base Transceiver Stations is preferably increased. It is, however, sometimes difficult to secure a place offering sufficient space to install a Base Transceiver Station. If a Base Transceiver Station is installed at a hard-to-reach place for a maintenance staff, it is not easy for him/her to maintain the Base Transceiver Station.
There is proposed a method by which narrow space or a hard-to-reach place is used effectively. According to the method, a radio part for performing radio communication with a mobile station is separated from the body of a Base Transceiver Station, and the radio part is installed at the narrow space or the hard-to-reach place.
A radio part is connected to the body of a Base Transceiver Station via a cable such as an optical fiber. The Common Public Radio Interface (CPRI) standards are proposed as an interface between the radio part and the body of the Base Transceiver Station. According to the Common Public Radio Interface standards, the radio part is called Radio Equipment (RE). The body of the Base Transceiver Station is provided with a Radio Equipment Control (or Controller) (REC) for controlling the Radio Equipment. The Radio Equipment and the Radio Equipment Controller are connected to each other via an optical fiber to perform communication with each other according to a predetermined protocol.
Further, there is proposed a method by which one Radio Equipment Controller is used to control a plurality of pieces of Radio Equipment. As described above, the installation location of Radio Equipment is more flexible as compared to the conventional technique. Accordingly, the length of a cable between the Radio Equipment Controller and each piece of Radio Equipment differs depending on the installation location thereof. This causes the difference in transmission rate in transmission paths between the Radio Equipment Controller and the individual pieces of Radio Equipment.
To cope with this, a method is proposed in which phase differences of transmission paths are determined, signal delays of the individual transmission paths are compared, and the signal delays are adjusted to one another.
As for the background art discussed above, the following documents should be referred to: Japanese Laid-open Patent Publication No. 2008-011498; and “Development of IP-compatible radio Base Transceiver Station”, published in NTT DoCoMo technical journal, Vol. 15, No. 1, issued on April 2007, edited by NTT DOCOMO, Inc., and written by Hidehiko OHYANE, Naoki NAKAMINAMI, Daisuke TANIGAWA, and Yoshitaka HIRAMOTO.
The Multiple Input Multiple Output (MIMO) technology has recently been proposed. The MIMO technology is to expand a band by combining a plurality of antennas with one another. A sending end transmits different types of data concurrently through the plurality of antennas. A receiving end receives the different types of data sent from the sending end through a plurality of antennas whose amount is the same as that of the antennas of the sending end, and combines the different types of data received. The band is, therefore, broadened artificially in accordance with the number of antennas.
The individual antennas of the receiving end are required to receive a plurality of pieces of data to be combined almost at the same time. This is because, if a delay occurs, the plurality of pieces of data received cannot be combined with one another. The sending side, therefore, is also required to transmit data through the individual antennas without any delay.
However, a conventional REC-RE type Base Transceiver Station such as that disclosed in Japanese Laid-open Patent Publication No. 2008-011498 is configured to cope only with a delay in a transmission path between a Radio Equipment Controller and each piece of Radio Equipment.